Fluidized bed combustion is a mature technology. Many fluidized bed processes where combustion occurs are known, including the regenerators associated with fluidized catalytic cracking (FCC) units, fluidized coal combustors, and "regenerators" associated with fluid cokers.
Many fluidized bed combustion processes achieve only partial combustion of carbon (in coke, hydrocarbon or coal) to CO.sub.2. Partial combustion, to CO, represents a loss of energy and a source of air pollution.
In FCC regenerators, it is known to add a CO combustion promoter, such as PT, to the circulating catalyst inventory. Adding 0.1-10, usually 0.5-2 wt ppm Pt is common in FCC processes to achieve complete CO combustion. The Pt makes the regenerator run hotter, because of the more complete CO combustion. More air is added per unit weight of carbon burned, because more CO.sub.2 is formed at the expense of CO. Although CO emissions are much reduced, there is an increase in NO.sub.x emissions, probably because of the more oxidizing atmosphere.
The Pt promoter lasts a long time in commercial FCC units, having an activity or catalyst life similar to that of the conventional FCC catalyst, which remains in the unit for months.
Similar results are noted in the Thermofor Catalytic Cracking (TCC) Process which is a moving bed analog to the FCC process.
Both FCC and TCC processes involve fairly clean feeds (heavy hydrocarbons) and stable, long lasting catalysts which are an ideal support for CO combustion promoters such as Pt.
Use of CO combustion promoters has been recommended for fluidized bed coke combustion. In U.S. Pat. No. 4,515,092 (Walsh et al), which is incorporated herein by reference, and in a related publication by Walsh et al entitled "A Laboratory Study of Petroleum Coke Combustion: Kinetics and Catalytic Effects", addition of sand-containing 0.1 and 1.0 wt. % Pt, is reported to promote CO combustion in a single fluidized bed of coke operating at 505.degree. C.
A recent development has been the commercialization of circulating fluid bed (CFB) boilers.
In CFB units, operation is complex. A fuel, usually a low grade fuel with a lot of sulfur and other contaminants, e.g. coal, is burned in a riser combustor. The flow regime is primarily that of a fast fluidized bed, i.e., there are no large "bubbles". Motive force for the fast fluidized bed is usually combustion air added at the base of the riser. There is usually an extremely large range of particle sizes in CFB units.
Combustion air is generally added to the base of the fast fluidized bed, and the resulting flue gas is discharged from the top of the fast fluidized bed, generally into a cyclone separator which covers most of the larger particles, typically 100 microns plus, while allowing finer materials (fly ash) to be discharged with the flue gas. Solids recovered by the cyclone are recycled into the fast fluidized bed.
Heat is removed from the CFB units in many places. CFB units take advantage of the extremely high heat transfer rates which are obtainable in fluidized beds, and provide for one or more areas of heat recovery from the fluidized bed. Most units have at least one relatively dense phase fluidized bed heat exchanger intermediate the cyclone separator solids discharge and the fast fluidized bed combustor.
Fluid flow in CFBs is complex because of the tremendous range in particle size of materials which must be handled by many CFBs. When coal is the feed to a CFB unit, the particle size distribution can range from submicron particles to particles of several inches in diameter.
Submicron to several micron particles present include fly ash, ground dolomite or limestone, and perhaps a few particles of ground coal.
Particles less than 100 microns in diameter usually have a short life in CFB units, because the low efficiency cyclones usually associated with such units must be able to let the fly ash out, while retaining essentially all of the 100+ micron material, which usually represents coal, or ground sulfur absorbing material such as dolomite.
The 100 micron-400 micron material in a CFB represents much of the circulating particulate inventory. Usually this material is the dolomite, limestone, and similar materials used as an SO.sub.x acceptor, and some portion of the low grade fuels such as coal. When clean, or at least low sulfur, fuels such as wood chips are burned the sulfur acceptor is not needed and sand, or some other inert is provided for fluidization.
The coal particles may range in size from several inches when first added to the fast fluidized bed to theoretically submicron particles produced by explosion or disintegration of large size particles of coal. The majority of the coal is in large particles, typically 300-1000 microns, which tend to remain in a lower portion of the CFB, by elutriation.
Many CFB units are designed to handle small amounts of agglomerated ash. At the temperatures at which CFBs operate (usually 1550.degree.-1650.degree. F.) there is much sintering of ash, which forms larger and larger particles. Many CFBs are designed to allow large ash agglomerates, typically in the order of 1000-2000 microns, to drop out of the bottom of the CFB unit or to be removed intermittantly.
The chemical reactions occurring during CFB operation are complex. Coke combustion, reactions of sulfur and nitrogen compounds with adsorbents, reactions of NO.sub.x with reducing gases (such as CO which may be present), etc., are representative reactions.
Despite the explosive growth in CFB technology (from no commercial units 1978 to about in operation or under construction 100 commercial units in 1988) I realized that the technology had some shortcomings. Particularly troublesome was the tendency of the units to all operate at the same exceedingly high temperature, which causes some metallurgical, operational and pollution problems. CFBs also operate with far more air than is required by stoichiometry.
Typical circulating fluidized bed designs are disclosed in U.S. Pat. No. 4,776,288 and U.S. Pat. No. 4,688,521, which are incorporated by reference.
Circulating fluid bed combustion systems operating with staged air injection, or staged firing, as disclosed in U.S. Pat. No. 4,462,341 or in a reducing mode circulating fluid bed combustion unit, such as disclosed in U.S. Pat. No. 4,579,070 will minimize somewhat NO.sub.x emissions. The contents of both of these patents are incorporated herein by reference.
Separation means used to remove recirculating solids from flue gas may comprise cyclones, or the gas and particle separation means disclosed in U.S. Pat. No. 4,442,797 which is incorporated herein by reference.
I reviewed the state of the art in circulating fluidized bed technology. Fortunately most of the work on circulating fluidized beds has been published in two volumes. The first was Circulating Fluidized Bed Technology, Proceedings of the First International Conference on Circulating Fluidized Beds, Halifax, Nova Scotia, Canada, November 18-20, 1985, edited by Prabir Basu, Pergamon Press (hereafter CFB I) and, more recently, by Circulating Fluidized Bed Technology II, Proceedings of the Second International Conference on Circulating Fluidized Beds, Compiegne, France, 14-18 March 1988, edited by Prabir Basu and Jean Francois Large, Pergamon Press (hereafter CFB II).
Other workers were aware of the problems remaining in use of CFB units, see e.g. Analysis of Circulating Fluidized Bed Combustion Technology and Scope For Future Development, Takehiko Furusawa and Tadaaki Shimizu, page 51, in CFB II. The authors focused on three areas.
1. Heat Recovery PA1 2. Design of Cyclones and Carbon Burn-Up PA1 3. NO.sub.x Emissions
I realized that the problems of better carbon burning, and reduced NO.sub.x /SO.sub.x emissions were related. This relationship can best be understood by reviewing the problem of emissions from CFB boilers.